add interactive BBQ knowledge atlas

[StevenGGA]

What changed

• Reworked the home page into a visual entry point for the encyclopedia.
• Added a 29-node interactive BBQ knowledge atlas connecting cuts, science, methods, and ingredients.
• Added search, domain filters, relationship highlighting, detail navigation, keyboard support, dark mode, mobile layout, and no-JavaScript fallback.
• Rebuilt the BBQ mastery path as a seven-stage visual route with browser-local progress tracking.
• Added a built-site link and fragment checker to CI and Pages deployment.

Why

The encyclopedia already had strong cross-domain content, but most of those relationships were hidden inside long-form prose and tables. This makes the knowledge graph explorable while preserving the existing evidence-aware documentation model.

Link reliability

Raw HTML links are not rewritten by MkDocs the way Markdown links are. During implementation this exposed would-be .md URLs in the visual components. Those links now use deployment-safe paths, and the new built-site checker prevents page and anchor regressions from reaching Pages.

Impact

Readers can enter by meat, mechanism, method, or ingredient; inspect direct relationships; follow real contextual links; and track their progress through the seven-part BBQ theory curriculum without an account.

Validation

• 18 pytest tests passed
• Ruff passed
• Strict content validation passed
• Metrics and experiment-index checks passed
• JavaScript syntax checks passed
• Strict MkDocs build passed
• 57 generated HTML pages and 5,418 internal/resource references audited with zero failures
• Playwright desktop and 375 px mobile interaction checks passed